How to save my apps somewhere when i update my iphone

Ok so i downloaded gta(its $5 )from my friends account and we don't really talk anymore i'm afraid to update my phone because ill lose it //please help

Added (1). IPHONE 4

Fist of all what kind and model phone you have?
From what i know once you sync your phone with iTunes it recognizes the apps that you have and saves the info on your computer and after you reset your phone it will transfer them back to your phone. If the app was purchased and added to your phone it will or should still be available after you update your phone